1. Field of Invention
The present invention relates to light strings and, more particularly, to decorative light strings employing LEDs.
2. Description of Related Art
Light emitting diodes (LEDs) are increasingly employed as a basic lighting source in a variety of forms, including decorative lighting, for reasons among the following. First, as a device, LEDs have a very long lifespan, compared with common incandescent and fluorescent sources, with typical LED lifespan at least 100,000 hours. Second, LEDs have several favourable physical properties, including ruggedness, cool operation, and ability to operate under wide temperature variations. Third, LEDs are currently available in all primary and several secondary colors, as well as in a “white” form employing a blue source and phosphors. Fourth, with newer doping techniques, LEDs are becoming increasingly efficient, and colored LED sources currently available may consume an order of magnitude less power than incandescent bulbs of equivalent light output. Moreover, with expanding applications and resulting larger volume demand, as well as with new manufacturing techniques, LEDs are increasingly cost effective.
LED-based light strings, used primarily for decorative purposes such as for Christmas lighting, is one application for LEDs. For example, U.S. Pat. No. 5,495,147 entitled LED LIGHT STRING SYSTEM to Lanzisera (hereinafter “Lanzisera”) and U.S. Pat. No. 4,984,999 entitled STRING OF LIGHTS SPECIFICATION to Leake (hereinafter “Leake”) describe different forms of LED-based light strings. In both Lanzisera and Leake, exemplary light strings are described employing purely parallel wiring of discrete LED lamps using a step-down transformer and rectifier power conversion scheme. These and all other LED light string descriptions found in the prior art convert input electrical power, usually assumed to be the common U.S. household power of 110 VAC to a low voltage, nearly DC input.
U.S. Pat. No. 5,941,626 entitled LONG LIGHT EMITTING APPARATUS to Yamuro (hereinafter “Yamuro”) briefly discloses that, although the sum of the (DC) LED voltage equals the source voltage, experience proves the circuit is unstable unless resistance is added. Yamuro then goes on to provide a method for calculating said necessary resistance. These and all other high-voltage LED light string descriptions found in prior art are fundamentally flawed in that they utilize the conventional, DC voltage ratings of the LED's.